Much of the time vitamin D is added to calcium supplements to aid the body with absorption. vitamin D was discovered in an attempt to find what vitamin was lacking in the cause of rickets.
Now easily synthesized vitamin D is added to many staple foods to fight the chance of rickets.
Milk is an excellent source of natural calcium and is now generally fortified with vitamin D for a one, two punch for strong bones.
Soymilk and some vegetable mixes are being fortified with both calcium and vitamin D to levels equivalent to milk for those who don't or can't drink milk.